The present invention relates to a color cathode ray tube, and particularly to a color cathode ray tube including an electron gun which is improved in resolution by enhancing focus characteristics over the entire phosphor screen and over the entire electron beam current region.
A color cathode ray tube used as TV picture tubes or monitor tubes at information terminals contains an electron gun for emitting a plurality (in general, three) of electron beams at one end of an evacuated envelope, a phosphor screen coated with a phosphor film of a plurality (in general, three) of colors on the inner surface of the other end of the evacuated envelope, and a shadow mask serving as a color selection electrode closely spaced from the phosphor screen, wherein a plurality of electron beams emitted from the electron gun are two-dimensionally scanned by magnetic fields generated by a deflection yoke provided outside the evacuated envelope, to produce a desired image.
FIG. 12 is a sectional view illustrating a configuration example of a color cathode ray tube to which the present invention is applied. In FIG. 12, reference numeral 21 indicates a panel portion; 22 is a funnel portion; 23 is a neck portion; 24 is a phosphor film; 25 is a shadow mask; 26 is a mask frame; 27 is a magnetic shield; 28 is a shadow mask suspension mechanism; 29 is an in-line electron gun; 30 is a deflection device; 31 is a beam adjustment device; 32 is an internal conductive coating; 33 is a tension band; 34 is a stem pin; and 35 is a getter.
In this color cathode ray tube, an evacuated envelope is formed of the panel portion 21, the neck portion 23, and the funnel portion 22 connecting the panel portion 21 to the neck portion 23.
The panel portion 21 has on the inner surface thereof a display screen composed of the phosphor film 24 coated with phosphors of three colors. The neck portion 23 contains the electron gun 29 for emitting three in-line electron beams. The shadow mask 25 having a multiplicity of apertures therein or a parallel array of narrow stripes is spaced closely to the phosphor film 24 of the panel portion 21.
In addition, characters Bc, Bs indicate electron beams. The deflection device 30 is mounted in a transition region between the funnel portion 22 and the neck portion 23.
The getter 35 is supported at the end of a getter support spring with its one end fixed on a shield cup of the electron gun 29 for increasing the degree of vacuum in the evacuated envelope by evaporating and dispersing a getter material in the evacuated envelope. The getter 35 is welded to the shield cup during assembling of the electron gun.
The three electron beams emitted from the electron gun 29 are deflected in the horizontal and vertical directions by vertical and horizontal deflection magnetic fields generated by the deflection device 30, are subjected to color selection through electron beam apertures in the shadow mask 25, and then impinge on respective phosphors, to produce a color image on the phosphor film 24.
FIGS. 13A and 13B are schematic side views illustrating configuration examples of in-line type electron guns to be incorporated in the color cathode ray tube shown in FIG. 12, wherein FIG. 13A shows a so-called uni-potential type electron gun, and FIG. 13B shows a so-called bi-potential electron gun.
In FIG. 13A, reference character K indicates a cathode; 1 is a first grid (hereinafter, referred to as "G1 grid", and the same rule applies correspondingly to the following); 2 is a G2 electrode; 3 is a G3 electrode; 4 is a G4 electrode; 5 is a G5 electrode; 6 is a G6 electrode; 7 is a shield cup; 8 is a stem; and 9 is a beading glass In this electron gun, the facing ends of the G4 electrode 4 and the G5 electrode 5 form a pre-main lens, and the facing ends of the G5 electrode 5 and G6 electrode 6 form a main lens.
In FIG. 13B. reference character K indicates a cathode; 1 is a G1 electrode; 2 is a G2 electrode; 103 is a G3 electrode; 104 is a G4 electrode; 7 is a shield cup; 8 is a stem; and 9 is a beading glass. In this electron gun, the facing ends of the G3 electrode 103 and the G4 electrode 104 form a main lens.
For a color cathode ray tube including at least an electron gun composed of a plurality of electrodes for accelerating and focusing three in-line electron beams, a deflection device for deflecting the electron beams in the horizontal and vertical directions, and a phosphor screen composed of a phosphor film which luminesces when the electron beams impinge thereon, various improvements have been made to obtain a desired reproduced image on the phosphor screen over the region extending from the center to the peripheral portions.
For example, Japanese Patent Publication No. Sho 53-18866 discloses a color cathode ray tube in which an astigmatic lens is provided in a lens region formed by a G2 electrode and a G3 electrode; Japanese Patent Laid-open No. Sho 51-64368 discloses a color cathode ray tube in which each of electron beam apertures in a G1 electrode and a G2 electrode of an in-line three-beam type electron gun is vertically elongated, the shapes of the electrodes are different from each other, and the ellipticity of the center beam electron beam aperture is smaller than that of the side electron beam aperture; Japanese Patent Laid-open No. 60-81736 discloses a color cathode ray tube in which at least one non-axially symmetric lens is formed of slits provided in a G3 electrode of an in-line type electron gun on the cathode side, the depth of the slit along the tube axis being larger for the center electron beam than the depth of the slit for the side electron beam, wherein electron beams are made to impinge on a phosphor screen via the nonaxially-symmetric lens; and Japanese Patent Laid-open No. Sho 57-151153 discloses a color cathode ray tube in which three apertures corresponding to three electron guns in a first grid electrode or a second grid electrode are configured that the areas thereof are equal to each other, and the diameter of the side beam apertures (side electron guns) is larger than that of the center beam aperture (a center electron gun) in the direction perpendicular to the in-line direction of the three beams.
The focus characteristics required of an in-line three-beam color cathode ray tube are improvement in resolution of images formed by three electron beams over the entire phosphor screen and over the entire electron beam current region in consideration of the luminous efficiency and luminosity factor of phosphors of three colors.
The design of an in-line electron gun capable of satisfying such requirements requires a high level technique.
To meet the above-described requirements of an in-line three-beam color cathode ray tube, the focus characteristics of three electron beams are required to be based on a good balance of the diameter of a main lens, the spherical aberration of a prefocus lens system, astigmatism correction, effects of an electron beam control portion, and the like. Also it is known that the diameter of a main lens is desired to be larger for improving the focus characteristics.
Furthermore, if the diameters of main lenses for three electron beams are to be increased as much as possible in a neck portion of a given diameter of a cathode ray tube, part of electric fields of the main lenses should be shared by the three electron beams, so that it becomes difficult to equalize the diameter of the main lens of a center electron gun to the diameter of the main lens of the side electron guns.